The present invention relates to methods of making diagnostic and therapeutic radiology equipment and more particularly, to improved methods for making cathode assemblies used in x-ray generating equipment, such as computerized axial tomography (C.A.T.) scanners. More particularly, the invention is directed to methods for making improved x-ray tube cathode cup assemblies including a solid, one piece insulator member.
Each x-ray tube is normally enclosed in an oil-filled protective casing. A glass envelope contains a cathode plate, a rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the tube proximate to the rotor and overlapping therewith about two-thirds of the rotor length. The glass envelope is enclosed in an oil-filled lead casing having a window for the x-rays that are generated to escape the tube. The casing in some x-ray tubes may include an expansion vessel, such as a bellows.
X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. To release electrons, the filament in the cathode is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the anode, whereby they are abruptly slowed down. The anode, usually referred to as the target, is often of the rotating disc type, so that the electron beam is constantly striking a different point on the anode perimeter. The x-ray tube itself is made of glass, but is enclosed in a protective casing that is filled with oil to absorb the heat produced. High voltages for operating the tube are supplied by a transformer. The alternating current is rectified by means of rectifier tubes (or "valves") in some cases by means of barrier-layered rectifiers.
For therapeutic purposes--e.g., the treatment of tumors, etc.--the x-rays employed are in some cases generated at much higher voltages (over 4,000,000 volts). Also, the rays emitted by radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles (for instance produced by a betatron), are used in radio therapy.
X-ray tube performance can be affected by the alignment of the filament in the cathode assembly. Specifically, during x-ray tube manufacturing, it is important to be able to initially align the filament and have it stay aligned during completion of the manufacturing cycle and during operation of the x-ray tube.
Previously, coiled tungsten filaments used in x-ray tubes were assembled and then aligned in the cathode cup. Once assembled, the filaments were heated to about 2800.degree. C. to produce the desired microstructure. During this heating, when assembled in the cathode cup, many filaments sagged and thus move out of alignment making it necessary to reseat them in the cathode cup and repeat the flashing or the heating to 2800.degree. C. In some instances, this step had to be repeated up to as many as five (5) times until the filament alignment in the cathode cup was obtained with the desired microstructure.
In the prior method of positioning the insulators in the cathode cup, prior to inserting the filaments in the insulators, an operator would cut each insulator flange according to a predetermined patterned which was dependent upon whether the x-ray tube was a single, double, triple, etc. filament x-ray tube. The resulting flange may, depending on the particular tube receiving the cathode cup, have developed burs as a result of the cutting process. These burs often resulted in the insulation being twisted or misaligned when the insulation flange was connected, such as by welding, to the surface of the cathode cup. Misalignment of the insulation in the cathode cup has resulted in filament misalignment.
Because a misaligned insulator could produce a misaligned filament thereby causing increased manufacturing costs, the need for new methods which provide properly aligned coiled tungsten filament(s) in the x-ray tube cathode cup became apparent. Thus, there is a need for a new method of assembling a one-piece insulator made of, for example, a ceramic material fixed on the cathode cup that is sized to slide into the cathode cup in a position such that the apertures formed in the one-piece insulator and the cathode cup are in proper alignment thereby resulting in proper alignment of the filament. The method should desirably provide for assembly of a one-piece unit and the cathode cup such that the resulting cathode has at least one filament with proper alignment upon completion of the manufacturing process.